1. Field of the Invention
The present disclosure relates to power line communications, and more particularly, to system that provides an interface between a power line and a communication device, such as a modem.
2. Description of the Related Art
Power line communications (PLC), also known as Broadband over Power Line (BPL), is a technology that encompasses transmission of data at high frequencies through electric power lines, i.e., conductors used for carrying a power current. Power current is typically transmitted through power lines at a frequency in a range of 50-60 hertz (Hz). In low voltage lines, power current is transmitted with a voltage between about 90 to 600 volts. BPL may also be carried out over medium voltage lines, operating in a range of 1,000 to 35,000 volts. The frequency of the data signals is greater than or equal to about 1 megahertz (MHz), and a voltage of the data signal ranges from a fraction of a volt to a few tens of volts. Data communication can employ various modulation schemes such as amplitude modulation, frequency modulation, pulse modulation or spread spectrum modulation.
A modem, used as part of a PLC network, may receive its electric power from a low voltage power line. Power line terminals on the modem may also be used for the transmission and reception of PLC signals.
A PLC modem may be included in communications node that is installed on an electric pole and powered from overhead lines. Lightning and other transients on such lines may have amplitudes in excess of those found on low voltage power line outlets inside buildings. Therefore, a power input circuit of the node needs to protect node circuitry from high voltage transient surges, e.g. 6,000 volts.
Surge protection components are often shunt components having substantial capacitance that would short circuit high frequency data signals entering or leaving the node on its power line. A power input circuit must simultaneously provide low loss high frequency data signal flow and adequate surge protection for all power ports and data ports.
A further requirement for overhead communications nodes is for remote diagnosis of faults, including loss of input power or fuse opening. The node's power input terminals also serve as sensor terminals for these conditions, and so receive the brunt of surge transients. For communications nodes that have backup battery power, information from a sensor can be transmitted to a central operations facility, from which maintenance personnel may be dispatched.
A node will generally require only a phase and a neutral conductor for node power. However, overhead power lines often have two or three phases, and it is useful to drive all of these phases with PLC signals. Driving all of the phase lines with the same PLC signal may increase electromagnetic emission from these lines. In such cases, it is preferable to drive different phase conductors with PLC signals of mutually opposing phases, so as to obtain some cancellation of emissions.
There is a need for a power line interface circuit for a PLC modem harmoniously integrates the aforementioned requirements.